Sulfamic acid

Production

Sulfamic acid is produced industrially by treating urea with a mixture of sulfur trioxide and sulfuric acid (or oleum). The conversion is conducted in two stages:

OC(NH₂)₂ + SO₃ → OC(NH₂)(NHSO₃H)OC(NH₂)(NHSO₃H) + H₂SO₄ → CO₂ + 2 H₃NSO₃

In this way, approximately 96,000 tons were produced in 1995.

Structure and reactivity

The compound is well described by the formula H₃NSO₃, not the tautomer H₂NSO₂(OH). The relevant bond distances are S=O, 1.44 and S–N 1.77 Å. The greater length of the S–N distance is consistent with a single bond. Furthermore, a neutron diffraction study located the hydrogen atoms, all three of which are 1.03 Å distant from nitrogen. In the solid state, the molecule of sulfamic acid is well described by a zwitterionic form :

Water solutions are unstable and slowly hydrolyze to ammonium bisulfate, but the crystalline solid is indefinitely stable under ordinary storage conditions. Its behavior resembles that of urea, (H₂N)₂CO. Both feature amino groups linked to electron-withdrawing centers that can participate in delocalized bonding. Both liberate ammonia upon heating in water.

Acid-base reactions

Sulfamic acid is a moderately strong acid, K_a = 1.01 x 10⁻¹. Because the solid is non-hygroscopic, it is used as a standard in acidimetry (quantitative assays of acid content).

H₃NSO₃ + NaOH → NaH₂NSO₃ + H₂O

Double deprotonation can be effected in NH₃ solution to give HNSO2−
3.

H₃NSO₃ + 2 NH₃ → HNSO2−
3 + 2 NH+
4

Reaction with nitric and nitrous acids

With HNO₂, sulfamic acid reacts to give N₂, while with HNO₃, it affords N₂O.

HNO₂ + H₃NSO₃ → H₂SO₄ + N₂ + H₂OHNO₃ + H₃NSO₃ → H₂SO₄ + N₂O + H₂O

Reaction with hypochlorite

The reaction of excess hypochlorite with sulfamic acid or a sulfamate salt gives rise reversibly to both N-chlorosulfamate and N,N-dichlorosulfamate.

HClO + H₂NSO₃H → ClNHSO₃H + H₂OHClO + ClNHSO₃H ⇌ Cl₂NSO₃H + H₂O

Consequently, sulfamic acid is used as hypochlorite scavenger in the oxidation of aldehydes with chlorite such as the Pinnick oxidation.

Reaction with alcohol

Upon heating sulfamic acid will react with alcohols to form the corresponding organosulfates. It is more expensive than other reagents for doing this, such as chlorosulfonic acid or oleum, but is also significantly milder and will not sulfonate aromatic rings. Products are produced as their ammonium salts. Such reactions can be catalyzed by the presence of urea.

ROH + H₂NSO₃H → ROS(O)
2O−
+ NH+
4

Applications

Sulfamic acid is mainly a precursor to sweet-tasting compounds. Reaction with cyclohexylamine followed by addition of NaOH gives C₆H₁₁NHSO₃Na, sodium cyclamate. Related compounds are also sweeteners, see acesulfame potassium.

Sulfamates have been used in the design of many types of therapeutic agents such as antibiotics, nucleoside/nucleotide human immunodeficiency virus (HIV) reverse transcriptase inhibitors, HIV protease inhibitors (PIs), anti-cancer drugs (steroid sulfatase and carbonic anhydrase inhibitors), anti-epileptic drugs, and weight loss drugs.

Cleaning agent

Sulfamic acid is used as an acidic cleaning agent, sometimes pure or as a component of proprietary mixtures, typically for metals and ceramics. It is frequently used for removing rust and limescale, replacing the more volatile and irritating hydrochloric acid, which is however cheaper. It is often a component of household descaling agents, for example, Lime-A-Way Thick Gel contains up to 8% sulfamic acid and pH 2–2.2, or detergents used for removal of limescale. When compared to most of the common strong mineral acids, sulfamic acid has desirable water descaling properties, low volatility, low toxicity. It forms water-soluble salts of calcium and ferric iron.

Importantly, sulfamic acid is preferable to use in household in comparison to hydrochloric acid for its intrinsic safety. If erroneously mixed with hypochlorite based products such as bleach, it does not form chlorine gas, where the most common acids would; the reaction (neutralization) with NH₃, produces a salt as depicted in the section above.

It also finds applications in the industrial cleaning of dairy and brew-house equipment. Although it is considered less corrosive than hydrochloric acid, corrosion inhibitors are often added to commercial cleansers of which it is a component. Some of its domestic use, e.g. Easy-Off, for descaling include home coffee and espresso equipment and in denture cleaners.

Other uses

Catalyst for esterification process

Dye and pigment manufacturing

Herbicide

Coagulator for urea-formaldehyde resins

Ingredient in fire extinguishing media. Sulfamic acid is the main raw material for ammonium sulfamate which is a widely used herbicide and fire retardant material for household product.

Pulp and paper industry as a chloride stabilizer

Synthesis of nitrous oxide by reaction with nitric acid

The deprotonated form (sulfamate) is a common counterion for nickel(II) in electroplating.

Silver polishing

According to the label on the consumer product, the liquid silver cleaning product TarnX contains thiourea, a detergent, and sulfamic acid.